Conventionally, molded products made of resin have been produced by injection molding apparatuses. In general, an injection molding device utilizes heat by a heater and shearing heat by rotation of a screw to heat pellet-like thermoplastic resin fed into a hopper to a molten temperature. In short, the thermoplastic resin is molten. The injection molding apparatus moves the screw forward to fill a cavity of a mold with the molten resin, the mold being clamped in advance. A pressure to the filled molten resin is kept by a forward movement force of the screw. The filled molten resin is cooled in the cavity to become a molded product. The mold is then opened, so that the molded product is ejected.
With regard to the aforementioned injection molding apparatus, the molten resin spreads in the cavity from a gate of the mold to an end of the cavity so that the molten resin fills the cavity. Meanwhile, the molten resin is cooled from a surface portion coming into contact with a cavity formation surface of the mold. Accordingly, a resin temperature decreases. In particular, this results in an increase in viscosity of a leading end of the spreading molten resin. As a result of the viscosity increase, the molten resin becomes less mobile. Therefore, there is a risk of appearance defects in molded products, such as weld marks, flow marks, blow-holes or defective transfer under a deficient injection pressure of the molten resin which is injected from a nozzle into the cavity.
Recently, techniques of injecting and molding large and thin molded products are required to meet need for further weight reduction of large automobile components made of resin such as bumpers, and a size increase and a weight reduction in frames for liquid crystal displays of liquid crystal televisions or the like. With regard to such large and thin molded products obtained by the conventional injection molding device, there is a risk of a short shot that the cavity is not filled with molten resin till the end because of a deficient injection pressure in addition to the aforementioned appearance defects.
An increase in an injection pressure may work for preventing the short shot. In this case, it is necessary to clamp a mold at a mold-clamping pressure against the injection pressure required for the injection molding. Therefore, it is necessary to prepare a large injection molding apparatus having a high mold-clamping pressure and a large mold endurable against the injection pressure and the mold-clamping pressure. Alternatively, an increase in gates or thickening a molded product may work for preventing the short shot. However, these techniques have various problems which are, for example, an increase in portions at which weld marks occur, and an increase in material costs.
Patent Document 1 discloses so-called heat-and-cool molding in which a mold is repeatedly heated and cooled. According to Patent Document 1, molten resin injected into a cavity is indirectly heated by heating the mold during the injection. Since a temperature of the molten resin is less likely to decrease, it is not necessary to increase an injection pressure and change the number of gates or the thickness of a molded product.
Recently, there has been a demand for techniques of providing molded products made of conductive resin required for electrostatic coating by mixing conductive filler with base insulative resin. Additionally, there has been another demand for techniques of injecting and molding a conductive material containing a resin material mixed with carbon fibers or the like in order to strengthen molded products.
As one of these techniques, Patent Document 2 shows electrifying a conductive molten material which flows in a passage of a nozzle of an injection molding apparatus. The molten material is directly heated by Joule heat immediately before filling a cavity of a mold.
With regard to the injection molding apparatus disclosed in Patent Document 1, the mold is heated, the mold having larger heat capacity than the molten resin to be filled. It takes a long time to cool the mold (i.e. to cool a molded product in the mold) after injection of the molten resin and pressure-keeping because the mold has a high temperature. Accordingly, this may lengthen an injection molding cycle from an injection process to an ejection process for a molded product.
With regard to the techniques disclosed in Patent Document 2, the molten resin injected from the nozzle into the cavity is cooled by the mold while the molten resin moves in the cavity, as described above. Therefore, the problems of the appearance defects or the like are not solved, the problems being caused by a reduction in fluidity.
The aforementioned problems are also seen in injection molding apparatuses for injecting and molding a metal material of aluminum or the like in a mold such as a die casting machine.